Gazelle Trim Trophy, EG Bates Track Bike, HR Bates Cantiflex bike, Nigel Dean fixed gear conversion, Raleigh Royal, Falcon Westminster.
70's u-frame bike. Thousands were made by all sorts of brands, they're all largely the same. A bit heavy, a bit flexy (specially at the frame joint), but perfectly useable. Don't expect it to be as nice to ride as a modern folder, but I would use one, under the right circumstances. In the UK, however, you could pick one up for £5-10 ($10-20), and that would be a 3 speed. $100 sounds a bit steep.
Wanted - SRAM/Sachs Elan 12 shifter and clickstick